Packaging is one of the final steps in the process of manufacturing integrated electrical semiconducting circuit components or chips. In packaging, a fabricated semiconductor chip is mounted in a protective housing. After packaging, the assembled component is subjected to final testing and then connected to an electronic circuit.
Currently, many semiconductor chips are contained in plastic packages. These packages are provided with reinforced metal leads for electrically connecting the chip to the printed circuit board which contains the circuit in which the chip is to be included Within the package, one end of each lead is connected to a specific bonding point on the chip, usually by an intermediate lead; the other end of the lead, which extends outside of the package, is attached to a connection on the printed circuit board.
Recently, advances in semiconductor manufacturing technology have made the fabrication of Very Large Scale Integration (VLSI) chips possible. VLSI chips comprise a large number of individual circuit components that are fabricated together on a single, very small chip. VLSI chips are able to perform a large number of electrical functions, and perform them more rapidly, than was previously possible.
To date, it has been difficult to provide suitable packaging for VLSI chips. One reason for this is because each VLSI chip requires a large number of connections to external circuit elements. Many VLSI chips have 100 to over 300 bonding points, each of which must be individually connected to a lead for connection to external circuit elements. This is a difficult task to perform because of the space in which these connections must be made is very small and because the chips are relatively fragile devices and the connections to them must be made with a considerable amount of care.
Tape Automated Bonding (TAB) is one of the few practical methods that have been developed in order to provide electrical connections to VLSI chips. In TAB, a chip is bonded to a section of TAB tape that has a number of individual leads that are plated on a film of insulating material. The film, which serves as a support for the leads, has a center opening which the chip is mounted over. The TAB leads are arranged so they extend outward from a center opening of the film; and project beyond its outer perimeter. Thus, each TAB lead has an inner lead portion that extends into the center opening of the film and an outer lead portion that extends beyond the outer perimeter of the film. A chip is positioned over the film center lead opening and is bonded to the lead inner lead portions at its bonding points; the outer lead portions of the leads are then attached to the appropriate conductive elements, such as contact pads on a printed wire board. Since the film can support an almost limitless number of leads, TAB has been an especially effective means to provide electrical connections to the bonding points of VLSI chips. Moreover, TAB is a cost-effective means of providing electrical connection to the chips.
Another reason that, to date, it has been difficult to provide a package suitable for a VLSI chip is the need to include a means to efficiently dissipate the heat generated by the chip. A chip generates heat as a consequence of its consuming power in order to perform the function it is designed for. Heat dissipation is an important consideration in the design of a chip package because most chips should be operated at temperatures below 80.degree. C. If a chip becomes too hot, the semiconductor junctions, which form the basic electrical components within the chip, have a tendency to break down and the chip may malfunction.
Heat dissipation characteristics are an especially important consideration in packages designed to house VLSI chips. This is because VLSI chips consume more power than their predecessors, and as a result, generate more heat. Therefore, a package in which a VLSI chip is housed should include some means to efficiently extract the heat, or otherwise cool the chip therein in order to insure it consistently functions properly.
There have been a number of chip packages developed which provide some path, such as a cooling fluid or a heat spreader, through which heat can be diffused away from the chips therein. However, many of these packages are designed so that only reinforced contact pins can be used to provide the needed electrical connections between the external circuit and the chips within the packages. In order to mount the package with contact pins on a circuit board, complementary plated-through holes are needed on the circuit board. The plated-through holes extend through the circuit board in which they are formed, and as a result, the conductors on the circuit board must be designed around them. This may be difficult task if the circuit board contains a large number of conductors, as is required by most modern electronic circuits.
One type of semiconductor package, designed for individual chips, that has good heat dissipation characteristics is the cerquad package. A cerquad package is formed of ceramic with a base that includes a raised shelf around its perimeter that defines a seating space where the chip is housed. A lead frame, formed of spaced apart reinforced metal leads, is embedded between the shelf, above where the chip is seated, and a ceramic frame disposed over the base. Each metal lead has an interior portion which projects into the interior of the package and an exterior portion which extends out from the package. A lid is secured to the frame over the seating space so as to complete the package and protect the chip therein.
A chip is assembled inside a cerquad package by first securing the chip in the seating space by a process known as die attachment. The bonding points on the chip are then electrically connected to the appropriate leads by wires that are individually attached therebetween. The lid is sealed over the seating space to complete the assembly process. In the assembled cerquad package, the metal leads function as conductive paths between the chip and the associated components of the electronic circuit the chip is connected to.
Cerquad packages have good thermal conductivity characteristics, that is heat readily transfers through them. A cooling fin assembly can readily be attached to the outer surface of the base of a cerquad package, so the heat transferred thereto from the chip can be rapidly diffused into the external environment. Cerquad packages can be readily manufactured to contain a single chip. Also, cerquad packages have leads that enable them to be easily surface mounted to the circuits which the chips they house are designed for. This eliminates the need to provided plated-through holes on the circuit board for electrically connecting the chip to the circuit, and the attendent need to design the circuit board conductors around the plated-through holes. Moreover, cerquad packages are very economical to manufacture. Thus, it is often desirable to package a stand-alone chip with significant heat generating characteristics in a cerquad package.
To date, however, it has been difficult to house a VLSI chip inside a cerquad package. This is in part due to the problems associated with connecting the bonding points of a VLSI chip to the package leads that connect the chip to the other elements of the circuit it is attached to. Wire bonding of the individual bonding points to the individual leads is a time consuming and expensive process that is prone to error because of the large number of leads and small spaces involved.
It has also been very difficult to house a chip with TAB leads inside a cerquad package. In part this is because the cerquad lead frame inevitably warps when it is embedded in the package. As a result, the lead frame leads are uneven and the outer lead portions of the TAB leads cannot be attached thereto by automated processes, such as one referred to as "gang bonding" which require the attachment be preformed on an even surface. The TAB lead outer lead portion must thus be individually attached to the lead frame leads. This is a costly process that is prone to error.
Moreover, glass is normally used to seal the frame to the base of a cerquad package so as to embed the lead frame therebetween. In order to apply the glass to the package, the glass's temperature must be raised to its flow point, which typically is 400.degree. C. Problems arise because TAB film starts to delaminate at approximately 300.degree. C. As a result of the delamination, the TAB leads warp, cross, and become loosened from their bonding points on the chip and from the cerquad package leads so as to render the assembled package useless. Thus, it has been very difficult to attach a VLSI chip, or any other TAB bonded chip, in a cerquad package.